1. Field of the Invention
The present invention relates to recovery boiler furnace safety appliances. More specifically, the invention is directed to safe operating methods and apparatus for paper pulp mill recovery furnaces adapted to burn concentrated black liquor.
2. Description of the Prior Art
Pursuant to present-day paper pulp mill operations, raw wood is delignified by a thermo-chemical process comprising an approximately 350.degree. F. cook in the presence of sodium hydroxide, sodium carbonate, sodium sulfide and other sodium based compounds. Under such conditions, the lignin binder in the raw wood matrix which holds the natural cellulose fibers together reacts with the sodium compounds to form water soluble lignin-sodium complexes thereby permitting a water wash separation of the black, tar-like lignin from the pulp for manufacture of bleached white paper.
Although the sodium compounds used in the aforedescribed process are relatively inexpensive, the quantities consumed in the 1500 tons of dry pulp per day production of an average pulp mill necessitate an economical recovery and re-cycle of the chemical values used for wood pulping. Moreover, such sodium-lignin complexes contain sufficient heat value and volatility to contribute favorably to the overall mill heat balance. These characteristics are combined in the liquor recovery furnace by fueling a boiler furnace with a concentrated flow stream of the spent or black pulping liquor. Combustion of the lignin fraction generates sufficient heat to evaporate the residual water vehicle and heat the steam required for the primary evaporative liquor concentration process. The residual ash, predominately sodium carbonate, falls to the furnace bed as a viscous smelt. Such smelt is cooled and dissolved in water to form the green liquor makeup stream from which the other fresh cooking liquor compounds are made.
Economics and thermal efficiency highly favor the use of such recovery furnaces but the practical, daily operation is critical and may be hazardous due to the explosive potential of a sodium-water reaction. Liquid sodium reacts to a combination of free water with explosive violence.
The presence of water containing boiler tubing above the furnace combustion chamber creates the greatest potential for such an explosion. As in any steam generating furnace, the boiler tubes are constantly subjected to high temperature and pressure stresses. In addition, however, such tubes in a sodium recovery furnace are subjected to a corrosive combustion atmosphere. Consequently, water leakage from the tubes is an ever present danger which occurs periodically as an operative fact and must be accommodated by appropriate safety procedures when it occurs.
Another necessary but hazardous operating circumstance arises from the viscous flow characteristics of the concentrated, 60% (plus) solids black liquor fuel that is consumed by such recovery furnaces. Such concentrated liquor fuel supply must be maintained above a certain temperature to be pumped and properly nebulized from spray nozzles for combustion. Consequently, the nozzle bearing spray guns are supplied from an externally heated circulation loop. Even so, in the course of normal operation, internal pipe walls of the circulation loop become coated with liquor deposits and, if allowed, will completely choke the passage. Accordingly, such circulation loop piping must periodically be purged with wash water. It is during such purging cycles that opportunity arises for an accidental discharge of purge water into the furnace smelt from an open liquor spray gun remaining in a firing port.
Still another hazardous operating circumstance relating to black liquor recovery combustion involves the critical balance of liquor solids and the heat available therefrom. Such liquor is derived from diverse pulp processing steps starting with the digester blow tanks and finishing with the pulp washers. The combination of aqueous residuals from such diverse steps is highly diluted and contains only about 5% solids. Accordingly, considerable concentration of the dilute liquor must occur before a combustible consistency is attained. Such concentration is a continuous evaporative process subject to numerous, critically balanced variables which occasionally fails to achieve the necessary 60 to 70% solids content. If the lignin/solids concentration in the black liquor fuel stream falls so low as to preclude sufficient lignin fuel content to evaporate all the water vehicle thereof, free water is available for direct, reactive contact with the sodium smelt in the furnace bed. When such conditions are allowed to continue, furnace explosions can and have occurred. For this reason, the solids content of the concentrated liquor to the spray guns is constantly monitored by means such as refractometer instruments. When the monitoring instruments detect a less than acceptable solids consistency, the pumps for the liquor spray supply loop are automatically stopped and automatic valves therein closed to preclude continued flows of such excessively dilute liquor to the spray guns.
Such response to a low liquor solids consistency condition meets the immediate explosion problem but creates a secondary problem of the supply loop line plugging as the liquor contained therein begins to cool. Consequently, under such conditions, the normal procedure is, as in the case of periodic line purge operation, to remove all liquor spray guns from their respective gun ports in the furnace wall and restart the circulation pumps so as to displace the low solids liquor in the liquor circulation piping system and to maintain sufficient fluidizing heat.
Whether prompted by periodic purge circulation or by a low liquor solids alarm, removal of liquor spray guns from respective furnace wall ports is a manual task. Unfortunately, such a recovery furnace has numerous liquor guns distributed about the firebox periphery. Consequently, the potential for oversight and failure to remove one or more guns is high. Moreover, the manual valves respective to each gun are subject to considerable handling abuse and tend to prematurely leak. A leaking gun tempts an operator to leave it in the port so he'll not have to cleanup the resulting floor mess.
It is, therefore, an object of the present invention to teach a method and apparatus which positively requires the removal of all liquor spray guns from a recovery furnace before the black liquor fuel supply circulation loop may be purged or resumed following a low consistency shutdown.
Another object of the present invention is to teach an apparatus for positively preventing the insertion if a liquor spray gun in a gun port during a periodic purge circulation interim or following a low consistency shutdown of the black liquor fuel supply system.